1. Field of the Invention
This invention relates to electrical energy storage. More particularly, a flexible dielectric film that can be a component of a high energy density capacitor that can operate at high temperatures and method for making are provided.
2. Description of Related Art
Capacitors with high volumetric energy density, high operating temperature, low Equivalent Series Resistance (ESR), and long lifetime are critical components for pulse-power, automotive, and industrial electronics. The physical characteristics of the dielectric material in the capacitor are the primary determining factors for the performance of a capacitor. Accordingly, improvements in one or more of the physical properties of the dielectric material in a capacitor can result in corresponding performance improvements in the capacitor component, usually resulting in performance and lifetime enhancements of the electronics system or product in which it is embedded. Since improvements in capacitor dielectric can directly influence product size, product reliability, and product efficiency, there is a high value associated with such improvements.
Certain improvements in capacitor dielectric materials can be considered as enabling to a particular technology application. For example, capacitors with high dielectric strength, low ESR, and low dielectric dissipation factor will allow high frequency or pulse-power applications to be reduced to a practical size. High temperature (>150° C.) operation will greatly simplify next-generation electric vehicles. Improved dielectrics will enable the specific power and reliability of switching power supplies, power conditioners, and filters to be increased. Improved energy density will decrease the area presently devoted to capacitor devices on printed circuit boards, reducing the weight and size of power conditioning systems, power supplies and down-hole tools for use in oil or gas wells.
To reduce the size of a capacitor while retaining all other physical and electrical characteristics, either an increase in the capacitor dielectric constant or dielectric breakdown strength is necessary. Both are fulfilled with the development of new thin, flexible dielectrics having high voltage breakdown strength (in excess of 20 kV/mil or 8 MV/cm), a high dielectric constant (greater than 2) and a low ESR loss (less than 0.1%). Some applications additionally require a stable dielectric constant with no reduction in lifetime at temperatures exceeding 150° C.
Progress on several fronts has recently been made in the semiconductor industry, where there has been rapid development of new, extremely thin, low-k and high-k dielectrics used in semiconductor devices. The advanced materials and processes have yet to migrate into the specialty or general use “macroscopic” capacitor industry, due mostly to their application in a focused niche, their relatively high production cost, and the inability of the dielectric material to be “rolled up” to produce large capacitance discrete capacitors. There is a need to apply some of the recent advances in semiconductor capacitor development, coupled with innovative materials and deposition processes, toward the construction of high volumetric energy-density rolled capacitors.
High energy density, high voltage non-polar capacitors are conventionally made using a metalized polymer film that is wound into a cylindrical shape. In conventional wound capacitors, the dielectric material is typically a polymer film. Common polymer dielectric materials include polycarbonate, polyester, polypropylene, polystyrene, and polysulfone. Polymer dielectric-based foil capacitors are generally fabricated by placing alternating sheets of polymer and metal foil in a stack and rolling the stack into a tubular shape or depositing a metal film on one side of the polymer then rolling two stacked metalized polymer films into a tubular shape. Electrical wires are connected to each metal foil. The dielectric material exists in the form of self-supporting layers that are thick enough to sustain the necessary operating voltage (typically at least 3-6 micrometers). Unfortunately, the large thickness of the polymer sheets reduces the energy storage density. Usually the dielectric constant of these capacitors changes and the lifetime is shortened at temperatures in excess of 100-150° C. due to deficiencies in the polymer material. Alternately, two polymer films coated with a thin layer of metal (usually 17-100 nanometers thick) are wound into a tubular shape to form a capacitor. The thin metal film has the advantage of clearing any short that may form if the polymer dielectric breaks down during operation. This may extend the life of the capacitor and minimize the chances of catastrophic failure of the capacitor.
Diamond-like carbon (DLC) or amorphous ceramics as the dielectric material on a metal foil has been proposed in U.S. Pat. No. 5,844,770, which is hereby incorporated by reference herein. Other recent patents that disclose capacitors include U.S. Pat. No. 6,894,887 (“Multi-layer capacitor and method for manufacturing the same”), U.S. Pat. No. 6,894,335 (“Thin film capacitor having multi-layer dielectric film including silicon dioxide and tantalum pentoxide”), U.S. Pat. No. 6,875,707 (“Method of forming a capacitor dielectric layer”), and U.S. Pat. No. 6,841,080 (“Multi-layer conductor-dielectric oxide structure”).
There is a specific need for flexible, high dielectric strength materials that can be formed into a capacitor with high breakdown voltage and high dielectric constant (resulting in high energy density) that operate at a higher temperature than a conventional capacitor. Methods for making the materials and integrating them into a capacitor structure are also needed.